The inventors have observed a need in the robotics industry for materials that are responsive to changes in adhesion, modulus, and mechanical damping. The development of responsive materials has been an intense area of research for over a decade. For example, the incorporation of multiple types of intermolecular interactions, including chemical cross-linking, physical cross-linking, hydrogen bonding, and metal co-ordination, have been previously explored for use in shape memory polymers. Shape memory polymers can be fixed into temporary shapes and return to their permanent shape through a temperature change. These materials are relatively rigid to provide some actuation and the necessary force to return to their original shape after deformation. However, shape memory gels typically have a modulus well into the 100 MPa range or are used in a stacked configuration with a stiffer material.
Due to the variety of potential commercial uses for reversible adhesives, many proposed approaches have been explored. Perhaps the most prevalent approach is bio-inspired adhesive approaches, often referred to as “gecko adhesion.” Gecko adhesion is the result of low strength Van der Waals forces that require a large amount of surface area to promote adhesion. As a result, the material approaches either involve very soft compliant materials or intricately structured substrates similar to the gecko foot-hair. Significant research has also been done utilizing adhesive proteins of the catecholic amino acid 3,4-dihydroxy-L-phenylalinine. All of these approaches require a significant mechanical force to remove the adhered substrate that will increase as the adhesive strength is improved. To decrease the pull-off force required to remove an adhesive, not necessarily bio-inspired, a number of strategies have been employed including a conversion from a sticky gel to a solution, chemically induced adhesive changes, and the use of differential expansive bleeding of poly(E-caprolactone). However, these approaches are not applicable to robotic systems due to issues with reproducibility cycles, implementation, and compliance, respectively. Reduction in pull off force has also been investigated using shape memory alloys in a bi-layer structure with a tack epoxy and in parallel plate geometry with hydrogen bonding groups between plates. However, these approaches do not address a change in adhesion only automated pull-off which requires additional energy to facilitate.
Current classes of materials that can vary their stiffness and damping include shape memory polymers, shape memory alloys, ionic gels, magneto-rheological fluids and electro-rheological fluids. For applications that require high damping, shape memory alloys are far too rigid. In contrast, magneto and electro rheological fluids are liquid, making them difficult to implement in many component geometries. Previous work on shape memory alloys and ionic gels has largely focused on stiffer, less compliant materials on the order of several MPa or higher.
Therefore, the inventors have provided improved polymeric materials having reversible mechanical and adhesive properties and methods of preparing polymeric materials having reversible mechanical and adhesive properties.